Photosensitization processes are used in a variety of medical applications to effect a therapeutic response in abnormal tissue. Photodynamic Therapy (PDT) is a developing approach for the treatment of cancer and other diseases which relies on the combination of light and drug (photosensitizer, PS) to produce reactive species, such as singlet oxygen (1O2), which are toxic to the target tissue. PDT in its current form is dependent on reactions of the PS with oxygen for its efficacy. Thus, hypoxia is currently a limitation to effective PDT. An understanding of subtle microenvironment effects on PS photophysics and photochemistry and development of strategies to reduce PDT- induced hypoxia or provide oxygen-independent phytotoxicity will aid in overcoming these limitations. The first specific aim addresses the influence of the biological microenvironment on the mechanism by which the PS can induce phototoxicity in cells, with specific focus on the role of the subcellular localization site of the PS. The influence of local PS concentration (which arises from confinement in small intracellular volumes) and oxygen concentration on photosensitization mechanism will be addressed. The relative susceptibility of the cell to damage at plasma membrane, mitochondria and lysosomes caused by both singlet oxygen and radicals will be determined, in addition to a quantitative comparison of phytotoxic potential of singlet oxygen and radicals. The second specific aim is the application of a non-invasive, spectroscopic, oxygen-sensing technique to monitor therapeutically- induced hypoxia in real time. The technique, based on oxygen dependence of the PS excited state lifetime, will be applied to investigate the effects of fluence rate on induction of hypoxia and to guide on-line, feedback control of light dosimetry. Onset of irreversible hypoxia, caused by vascular occlusion, will be investigated as a practical end- point for PDT treatment. The culmination of these studies will be a dynamic approach to PDT dosimetry which is unaffected by tumor to tumor variations between patients and capable of ensuring optimal therapeutic outcomes in the shortest possible treatment duration. The final specific aim is the development of a novel strategy for oxygen- independent photosensitization via radical generation on irradiation with low intensity red light, as used currently in PDT. The applicability of photoinduced intramolecular radical generators (PIRGs) to overcome hypoxia by producing reactive radicals under mild irradiation conditions will be determined. The ability of a PIRG to modulate between oxygen-dependent and independent photosensitization mechanisms in direct response to oxygen levels will be investigated.